Measuring subs only

[Mike-48]

I want to measure two subs that are ganged together. The goal is setting the best phase relationship between them. For that reason, I have turned the mains OFF to see only what's happening with the subs.

I am using REW 5.20.8 under Windows 10. The mic is a UMIK-2.

I have the PC with REW in the next room to protect my ears from the sweeps, and I am using REW-generated sweeps (10-800 Hz) in files, played (as any track is played) through my networked audio system. That also means I don't need to change any connections to the preamp.

HOWEVER, REW is not hearing the timing "chirp" and that seems to cause the measurement not to run. Of course, the subs don't respond high enough for the chirp. (I don't have the problem when I'm running full-range sweeps through the whole system.)

How can I work around this? If necessary, I could run a long SPDIF cable from pre to a USB converter straight to the laptop and so use REW as most seem to do. I would do that if necessary but would rather not.

Thanks!

 

[John Mulcahy]

If there is some form of bass management to handle the crossover between subs and mains easiest is to turn the mains back on and set the crossover to its highest setting. Worth noting, however, that sweeps do not need to be loud and for a sub measurement do not need to go as high as 800 Hz.

 

[sm52]

Mike-48
Each sub needs to be measured separately. This requires one channel of the sound card. Temporarily connect the network (speaker with HF driver) without a subwoofer to the second channel. And tell REW that this is the reference channel.

 

[Mike-48]

If there is some form of bass management to handle the crossover between subs and mains easiest is to turn the mains back on and set the crossover to its highest setting. Worth noting, however, that sweeps do not need to be loud and for a sub measurement do not need to go as high as 800 Hz.

Thanks, John. Is there no way to make these measurements in REW without the mains (and with the same DSP crossover settings I intend to use)? A second step will adjust timing (phase) between mains and the ganged subs. The goal of the first step is getting the two subs coordinated with one another at the main listening position, both so they can give the smoothest overall response and because they will be treated as one during the second step.

I appreciate your points about volume and frequency range of the sweeps.

 

[John Mulcahy]

No, not with a USB mic, as you wouldn't have a timing reference. You can measure them separately, as SM52 says, keeping the left channel (say) as the one with the timing reference output (without a measurement sweep) going to the left main and connecting the right channel (with the measurement sweep) to each sub individually. You can see the effect of relative timing and gain changes between the subs in REW using the Alignment tool. Example file settings:

 

[Mike-48]

Might I ask, is a timing reference always needed for REW to take the measurements with a USB mic? I can't see that it would be needed by my specific measurement otherwise. I'll be feeding the same signal to two subs and adjusting the phase relative to the other to get smoothest real-time response. I do not plan to use the alignment tool at this time (not saying it's not a better mousetrap).

Another question is why, when I enter the same parameters as you posted above, the sweep length is shown as 2.7 sec long, while in your example, the length is shown as twice that (5.5 sec).

 

[John Mulcahy]

If you play from file a timing reference is required to synchronise capture of the signal. If you play directly from REW you could remove the timing reference. The duration display is a bug, in some circumstances it can show the duration for the REW playback rate instead of the selected WAV rate so I guess you have the REW sample rate set to 96 kHz. I'll fix that for the next build.

 

[Mike-48]

If you play from file ...

Thanks, John. Very useful information. I'll run a long USB cable and hope it's close enough to spec to do the job.